Armor plate of hardened steel has been used for many years to provide protection of objects against damage. Vehicles such as tanks, military sites, vaults, and safes, etc. have used steel armor plate to provide such protection.
In order to increase the protection provided, it has previously been proposed to use spaced layers of steel. For example, U.S. Pat. No. 1,548,441 Branovich discloses an armor protected fuel tank wherein a layer of wood and a layer of semi-cured rubber are positioned between a steel tank and an outer armor plate. U.S. Pat. No. 2,348,130 of Hardy, Jr. discloses spaced metal plates between which a layer of rubber is positioned with pockets in the rubber filled with abrasive material such as sand. U.S. Pat. No. 2,733,177 Meyer discloses an elastic cascading impact absorber wherein layers of armor are spaced with respect to each other by elastic material which is disclosed in preferred embodiment as being formed sheet metal springs. U.S. Pat. No. 4,455,801 Merritt discloses a lightweight vault wall wherein layers of metal, stainless steel and aluminum, cover spaced layers of plywood adjacent each of which is provided a layer of expanded metal mesh that is spaced from the other layer of expanded metal mesh by a foamed plastic core.
Two different basic types of armor plate are conventionally utilized at the present time. One type is high-hard armor that is extremely hard and thus capable of preventing penetration of penetrating type of projectiles. The other type is rolled homogeneous armor that is somewhat softer than high-hard armor but is more ductile so as to prevent brittle fracture. Prior art references which disclose compositions and processing used in hardening of steel plates include: U.S. Pat. Nos. 774,959 Tresidder; 1,043,416 Giolitti; 1,079,323 Benthall; 1,097,573 Wales; 1,563,420 Johnson et al; and 1,995,484 Sullivan as well as the previously mentioned U.S. Pat. No. 2,733,177 Meyer.
In order to decrease weight, armor plate and the like have previously included holes such as illustrated by U.S. Pat. No. 3,763,838 of Butterweck et al which discloses a protective shielding for vehicles. While circular holes such as disclosed by Butterweck et al or slots are the easiest to produce in armor by punching, such shapes have ballistic gaps that reduce the protection provided. Similarly, square holes which will provide the lowest weight also have ballistic gaps that reduce the protection provided.
Other prior art references disclosing armor plate or the like include U.S. Pat. Nos. 45,536 Terwilliger et al; 874,729 DeBolula; and 4,178,859 Seiz et al.
DISCLOSURE OF INVENTION
An object of the present invention is to provide improved case-hardened plate armor and a method for heat treating steel plates to provide the armor.
The improved case-hardened plate armor according to the invention includes a steel plate that is heat treated to provide carbonitride surfaces and a tough, ductile core. The carbonitride surfaces of the steel plate have a hardness of at least 66 on the Rockwell C scale to prevent surface penetration, and the tough, ductile core is softer than the carbonitride surfaces to prevent brittle fracture of the steel plate. While the hardness of 66 on the Rockwell C scale of the carbonitride surfaces is adequate, it is preferable for the carbonitride surfaces to have a surface hardness of at least 67 on the Rockwell C scale to provide greater resistance to surface penetration.
Either rolled homogenous armor or high-hard armor may be utilized to provide the case-hardened plate armor with carbonitride surfaces. When rolled homogenous armor is utilized, the core hardness is in the range of 45 to 50 on the Rockwell C scale. When high-hard armor is utilized, the core hardness is in the range of 52 to 54 on the Rockwell C scale. In either case, the steel plate preferably has a thickness in the range of about 0.15 to 0.5 of an inch.
The plate armor with the carbonitride surfaces may be initially formed with holes prior to the heat treating in order to effect weight savings. However, it is also possible to utilize the carbonitrided steel plate without any holes if the weight savings is not necessary. When the holes are utilized, it is preferable to have the holes provided with the same size and shape as each other arranged in a repeating pattern. Most preferably, webs between the holes have a width in the range of 0.1 to 0.25 of an inch to provide best results in the provision of surface hardness and core toughness.
The method for case-hardening of the steel plate to provide the plate armor includes heating of the steel plate in an atmosphere of nitrogen and carbon, subsequently quenching the heated steel plate, tempering the quenched steel plate, deep freezing the tempered steel plate, and subsequently again tempering the steel plate after the deep freezing to provide hard carbonitride surfaces and a softer but tougher and more ductile core.
The steel plate is disclosed as being heated in an atmosphere of cracked ammonia and methane to provide the nitrogen and carbon, and such heating is preferbly performed for 1 to 3 hours at a temperature in the range of 1300.degree. F. to 1550.degree. F.. After such heating, the quenching is preferably performed by an oil quench to insure that all austensite is changed to martensite and to also prevent distortion as the quenching takes place.
The initial tempering of the quenched steel plate is performed for 1/2 to 2 hours at a temperature in the range of 275.degree. F. to 325.degree. F. This tempering changes the martensite to tempered martensite and ferrite.
An air cooling is preferably utilized after the initial tempering to eliminate the expenditure of unnecessary energy in effecting the deep freeze step. This deep freezing is preferably performed for 1 to 3 hours at a temperature in the range of -50.degree. F. to -150.degree. F. to change any retained austensite to martensite.
The final tempering after the deep freeze is preferably performed for 1/2 to 2 hours at a temperature in the range of 275.degree. F. to 325.degree. F., which are the same time and temperature parameters as the initial tempering. This subsequent tempering changes any additional martensite resulting from the deep freeze step to tempered martensite and ferrite.
Any holes in the steel plate are formed prior to the initial heating in the atmosphere of nitrogen and carbon to facilitate the hole formation in the steel prior to its hardening.
The objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.